Organic resin-covered metal sheets obtained by covering a metal sheet such as of aluminum with an organic resin have long been known as materials for producing cans. It has also been well-known to subject this laminated material to draw-working or draw-ironing to produce seamless cans for containing beverages, or to press-form the laminated material to produce can lids such as easy-to-open ends. For instance, an organic resin-covered metal sheet having, as an organic resin film, a thermoplastic resin film of a polyester resin comprising chiefly an ethylene terephthalate unit has been widely used as a material for producing seamless cans (patent document 1).
A seamless can made from the organic resin-covered metal sheet is filled with the content and is sealed. Here, if the seamless can is applied with an external shock (dented) as a result of falling or the like, the metal material may undergo deformation at that part. At the same time, the organic resin film may often be cracked due to the shock and deformation of the metal material. The portion where the organic resin film is cracked may cause corrosion. If the content filled therein is a strongly corrosive acidic beverage, then the can body may often be perforated due to the corrosion. It is, therefore, important that the can does not corrode despite it is applied with an impact or shock as a result of falling or the like. This property is called dent resistance. Excellent dent resistance has now been desired to cope with even the strongly corrosive contents.
As a metal sheet used as an organic resin-covered metal sheet for producing cans, on the other hand, there has generally been used a metal sheet having surfaces treated by conversion treatment in an attempt to improve corrosion resistance and adhesion (hot water-resistant adhering property) between the organic resin film and the metal material under high-temperature and wet environments such as of sterilization treatment after filling with a content and sealing. The surface treatment of this kind may be, for example, a treatment with chromium phosphate. The organic resin-covered surface-treated metal sheet comprising a metal sheet with its surface being treated with the chromium phosphate can be formed into a seamless can. When filled with the content, sealed and is subjected to the sterilization, the seamless can features excellent hot water-resistant adhering property between the organic resin film and the metal material and has, therefore, been widely used. From the viewpoint of environmental protection, however, it has been increasingly demanded to employ a chromium-free surface treatment. Moreover, the surface-treatment coating formed by the treatment with the chromium phosphate is a hard inorganic film which tends to be easily cracked during the forming treatment or when an external shock is given thereto. Namely, the surface-treatment coating formed by the treatment with the chromium phosphate is accompanied by a defect in regard to work followability and resistance against shocks. Despite of using the organic resin-covered surface-treated metal sheet obtained by treating the surface of the metal sheet with the chromium phosphate, therefore, there has not yet been obtained a seamless can that satisfies the above dent resistance.
So far, a number of chromium-free surface treatments have been proposed for the materials for producing cans. For instance, a conversion treatment of the organic/inorganic composite type using a zirconium compound, a phosphorous compound and a phenol resin has been proposed for the seamless aluminum cans covered with an organic resin, exhibiting hot water-resistant adhering property comparable to those attained by the treatment with the chromium phosphate (patent document 2).
Despite of using the above chromium-free surface-treated metal sheet, however, there has not yet been obtained a seamless can that satisfies the dent resistance.
Further, as a method of improving the corrosion resistance and close adhesion of the organic resin-covered surface-treated metal sheet having a chromium-free surface-treatment coating, there has been proposed an art of forming a primer coating under the organic resin film. For instance, there has been proposed a resin-covered aluminum alloy sheet obtained by covering an aluminum alloy sheet with a polyester film via a primer coating that comprises a polyester resin and a phenol resin, the aluminum alloy sheet forming thereon an organic/inorganic composite surface-treating layer that comprises a zirconium compound and/or a titanium compound and an organic compound (patent document 3).